Continual advancements in the speed and integration level of integrated circuits used in high performance systems have created a demand for the development of an interconnect technology that offers a high wiring density, good electrical characteristics for the propagation of high-speed signals, and good thermal performance. Multi-layer interconnection schemes with fine line conductors and associated ground planes have been proposed for applications in high performance systems. Fine geometry copper conductor lines defined in a photolithographically patterned layer of a low dielectric constant polymer, such as a polyimide, have emerged as a versatile packaging approach for the interconnection lines between densely packed integrated circuit chips in high performance systems.
In general, an initial X-plane of metallic features, such as fine geometry conductor lines, can be produced by the following steps: (1) depositing a film of a metallic seed layer on a dielectric substrate, (2) etching the seed layer to form fine geometry lines that serve as the electroplating base for the conductor lines, (3) spin coating a layer of a photosensitive dielectric composition over the dielectric substrate and etched seed layer, (4) photolithographically patterning the layer of the dielectric composition to form fine geometry dielectric features, the seed layer being uncovered between these dielectric features, and (5) forming the metallic features by electroplating a conductive material, such as copper, onto the seed layer between the patterned dielectric features.
In order to increase the density of the number of fine geometry conductor lines, an additional secondary Y-plane of conductor lines passing in a direction orthogonal to the lines of the initial X-plane can be formed on top of the initial X-plane. Electrical connections between the lines of the two planes are provided by via interconnects between the two planes. The via interconnects are holes that pass through a layer of the dielectric composition that is coated over the initial X-plane and serves as the base for the secondary Y-plane of orthogonal conductor lines. When certain conductor lines (e.g., copper lines) in the initial X-plane are coated with a dielectric composition that includes a polyimide precursor having a photosensitive functional group that renders the dielectric composition photosensitive, a chemical interaction between the photosensitive functional group of the polyimide precursor and the composition making up the upper surface of the conductor lines changes the photosensitivity of the dielectric composition in a manner that adversely affects the results of the photolithographic patterning step. Dielectric features, especially the via interconnects, that are photolithographically patterned into the altered layer of the dielectric composition are incomplete and less than well defined. The incomplete and less than well defined dielectric features are undesirable because of the disruptions or breaks they introduce into the conductive features that are electroplated between the patterned dielectric layers. These inconsistencies introduce nonreproducible electrical characteristics (e.g., electrical resistance and impedance) into the conductive features and may also result in electrical discontinuity in the electrical interconnection scheme.
Prior attempts to prevent this chemical interaction between the surface of the metallic conductor lines and the photosensitive functional group in the dielectric composition include coating a thin layer of titanium or chromium over the surface of the conductor lines prior to coating the surface with the photosensitive dielectric composition. Another method proposes to coat the metallic conductor lines with a layer of a nonphotosensitive polyimide before applying the photosensitive dielectric composition. The nonphotosensitive polyimide, like the thin layer of titanium or chromium, reportedly provides a physical barrier between the photosensitive functional group and the surface of the metallic conductor lines. The physical barrier serves to prevent the chemical interaction; however, these methods require additional process steps and increase the possibility of poor adhesion between the overlapping layers. Further, the use of the titanium, chromium, or nonphotosensitive polyimide layers requires that the protective layer be removed in order to allow a direct electrical connection to be made between the conductor lines of the upper secondary Y-plane and the conductor lines of the lower initial X-plane.